In general, electrolytic capacitors comprise anodes and cathodes that are separated by a porous separator material impregnated with an ionically conductive electrolyte. The electrolyte is typically composed of water, solvent(s), salt(s) of weak inorganic or/and organic acids. The anodes are of a valve metal having its exterior surface coated with a film of the corresponding oxide serving as a dielectric. Valve metals include and are not limited to aluminum, tantalum, niobium, titanium, zirconium, hafnium, and alloys thereof. The valve metals can be in any conventional form. Examples include etched foil, sintered powders, or other porous structures.
Anodizing the valve metals in an appropriate anodizing electrolyte forms the oxide film. The film thickness increases with the anodizing voltage. The desired oxide film thickness is determined by a capacitor working voltage, operation temperature and other performance requirements.
Maximum anodizing voltage and quality of the oxide formed strongly depends on the valve metal, the anodizing electrolyte composition, and the anodizing protocol. The anodizing protocol refers to a series of voltage/current “on” and “off” sequences.
It is believed that locally excessive temperatures and insufficient material transport in porous valve metal bodies during anodizing (especially for anodization of high voltage, large, pressed and sintered tantalum powder anodes) causes breakdown during anodization or poor anode electrical properties. There have been numerous attempts to solve these problems by improving the heat and electrolyte transport between the anodes and the bulk electrolytes. Some of the prior art methods include: controlling the anodizing current density; mechanical, sonic, or ultrasonic agitation of the electrolyte; anodizing by combining control of voltage/current and controlled rest steps (U.S. Pat. No. 6,231,993 to Stephenson et al.); and controlled pulses of the voltage/current (U.S. Pat. No. 6,802,951 to Hossick-Schott). These methods require sophisticated electronics for current/voltage/power control and frequent on/off switches that increase anodizing time. Additionally, it is believed that the eruptive increase in current/voltage in the case of pulsed anodizing may cause early breakdown and poor oxide quality.
A pressed tantalum powder pellet is a porous structure. During the prior art anodization process based on controlling the current density, the tantalum pellet is oxidized to a desired formation voltage by applying a current to the pellet. An example of this prior art protocol is illustrated in FIG. 1 where the current is maintained (line 2) and the power and voltage increases (line 4) over time. Such a simple anodizing protocol may be adequate for low voltage anodization where the breakdown voltage is intended to be less than about 100 volts. For high voltage anodization, i.e., greater than about 100 volts, as the anodizing voltage increases, the temperature in the porous valve metal anode increases. The locally excessive temperature in the anode promotes oxide defects, gray-out, and early anodizing breakdown. This traditional method has been confirmed in U.S. Pat. No. 6,802,951 to Hossick-Schott.
In the '951 patent, Hossick-Schott writes, “Traditional methods of forming the oxide layers are described in the prior art, e.g., in U.S. Pat. Nos. 6,231,993, 5,837,121, 6,267,861 and in the patents and articles referenced therein. Typically, a power source capable of delivering a constant current and/or a constant potential is connected to the anode slug that is immersed in the electrolyte. The potential is then ramped up to a desired final potential while a constant current flows through the anode-electrolyte system.”
An obvious variation of FIG. 1 was disclosed in the '951 patent. Hossick-Schott disclosed and claimed an anodization protocol having (1) the voltage rise to a predetermined level; (2) when the voltage rises the current remains constant, (3) when the voltage reaches the predetermined level, the current decreases; and (4) when the current and/or voltage are rising, being maintained or decreasing, the electrolyte composition is agitated.
An alternative anodization (formation) protocol for high voltage sintered tantalum anodes is disclosed by Stephenson et al. in U.S. Pat. No. 6,231,993. The '993 patent is assigned to Wilson Greatbatch Ltd., the assignee for this application. Stephenson et al. disclose (bracketed material added for clarity) the following anodization protocol, which is partially illustrated in FIG. 2:
An exemplary formation protocol for a sodium reduced tantalum powder pellet is as follows. Exemplary sodium reduction tantalum pellets are available from H. C. Starck Inc., Newton, Mass. under the “NH” family designation. In this exemplary protocol, the pellet has a weight of about eight grams and the desired target formation voltage is 231 volts. The formation electrolyte is of polyethylene glycol, de-ionized water and H3PO4 having a conductivity of about 2,500 μS[/cm] to about 2,600 μS[/cm] at 40° C. The formation protocol is as follows:
1. The power supply is turned on at an initial current [line 2] of 80 mA until the voltage reached 75 volts. The power supply is then turned off for about three hours.
2. The power supply is turned back on at 80 mA, 75 volts and to 115 volts. The power supply is then turned off for about three hours.
3. The power supply is turned back on at 49 mA, 115 volts and to 145. The power supply is then turned off for about three hours.
4. The power supply is turned back on at 49 mA, 145 volts and to 175. The power supply is then turned off for about three hours.
5. The power supply is turned back on at 40 mA, 175 volts and to 205. The power supply is then turned off for about three hours.
6. The power supply is turned back on at 36 mA, 205 volts and to 225. The power supply is then turned off for three hours.
7. The power supply is turned back on at 36 mA, at 205 volts and to 231. The pellet is held at 231 volts for about one hour to complete the formation process. The anodized pellet is then rinsed and dried.
If desired, the formation process is periodically interrupted and the anodized pellet is subjected to a heat treatment step. This consists of removing the anode pellet from the anodization electrolyte bath. The anode pellet is then rinsed and dried followed by heat treatment according to the procedure described by D. M. Smyth et al., “Heat-Treatment of Anodic Oxide Films on Tantalum”, Journal of the Electrochemical Society, vol. 110, No. 12, pp. 1264-1271, December 1963.
The anodization protocol illustrated in FIG. 2 controls the current and decreases the heat generated in comparison to the protocol illustrated in FIG. 1. By decreasing the temperature rise, the FIG. 2 anodization protocol obtains an anode having decreased DC leakage. However, as with any protocol there is a desire to further improve the quality of the anodized valve metal. One way to measure the improved quality of an anodized valve metal is to determine if the DC leakage decreases. A decreased DC leakage indicates a better oxide formation on the valve metal and more stable performance of the subsequently built capacitor. Better oxide formation, in turn, is obtained by better heat dissipation in the valve metal during anodization.
In that respect, the present invention teaches a method of anodization that simplifies the equipment and process, reduces anodization time, and provides a better quality oxide. Although this invention is, in principle, applicable to all valve metal anodes, it is particularly useful for anodizing a high voltage sintered tantalum structure.